The Sainsbury family is in a bitter dispute with Prince Charles' flagship charity over its use of a £2.5m bequest intended to help young people from poor rural communities find work. Annabel Kanabus, the scion of one of Britain's wealthiest dynasties, has demanded that the Prince's Trust repay £1m and waive its right to a further £1.5m bequeathed to it by her son, Jason Kanabus, who died suddenly from cancer last summer, aged 30.

The executors of the son's estate are angry that, almost a year after the Prince's Trust took control of the first £1m, it has provided no evidence that beneficiaries have gone on to find work in farming.

The Prince's Trust denies misusing the money and refuses to repay it. The trust has also demanded that the balance of £1.5m is paid to it immediately.

Jason was the grandson of Sir Robert Sainsbury, the supermarket baron and a patron of the arts who owned a country mansion in Berkshire filled with drawings by Picasso, Degas and Giacometti. On his 30th birthday, just three weeks before his death, Jason received a £2.5m inheritance from his grandfather's fortune. Soon afterwards doctors told him he had just weeks to live and the next day he collapsed in the sitting room of his farmhouse and died.

Shortly before he died, Kanabus chose the Prince's Trust to administer his legacy, partly because he admired the support its president, Prince Charles, had given to rural communities. The prince has backed employment schemes for young people in the Yorkshire Dales and Humberside, he farms organically at Duchy Home Farm in Gloucestershire and his website says he "cares deeply about the welfare of those in the British countryside".

Despite the wealth of Kanabus's extended family, estimated at £2.1bn by the Sunday Times Rich List, Jason worked on the family's 400-acre estate in Horsham, West Sussex. His mother had given away a large part of her £3m fortune when she was 30 to set up an Aids charity. Her son wanted to do likewise.

He drew up a will which included a "request" that his wealth should be used "to help young people become established in farming". On the advice of his solicitor, he used language that was not legally binding, to allow the benefiting charities to act in the spirit of the bequest.

Now, eight months after sending the first £1m of Jason's legacy to the Prince's Trust, the executors have demanded it back, accusing the trust of failing to follow the wishes of the deceased.

In a letter to Sir Fred Goodwin, the chairman of the trust, Jason's father Peter wrote: "[The Jason Kanabus Fund] continues either to accumulate unspent interest and/or to benefit other young people, but it is not helping young people to become established in farming." Mrs Kanabus said she was "devastated" that the legacy was not being spent as envisaged. "We asked them how many people have gone on to jobs in farming as a result of the £50,000 spent and they said none. We have asked for our money back."

She claimed that at a meeting with the executors of her son's will in March 2007, Martina Milburn, the chief executive of the Prince's Trust, said the funds would be used to help young people beyond the four categories of disadvantage that the trust normally targets: the unemployed, educational under-achievers, people leaving care and offenders or ex-offenders.

"They are not going to find in those groups young people who are struggling in farming, and those are the people Jason wanted to help," Kanabus said. His goal had been to help young people in poor rural areas set up farming businesses, for example, to train struggling young farmers as crop sprayers, allowing them to work as contractors or set up small businesses.

"It seemed to get to the point where you could only qualify for help from Jason's legacy if you got yourself in trouble with the law," she said. "It is no consolation the money is helping young people in general. He wanted to help young people in farming."

The Prince's Trust denies departing from any agreement with Kanabus, or the terms of Jason's will. "The trust has given almost 40 disadvantaged young people the chance to get into the farming industry, thanks to the Jason Kanabus Fund," a spokesman said. "Three in four of the young people helped by the fund have moved into work, education or training."

Eleven took work placements on Welsh farms, seven are learning to propagate apples in Suffolk, and in Gloucestershire others took an introductory course in farmwork, forestry, ground-maintenance and blacksmithing.

Milburn said the trust was "extremely disappointed" at the executors' allegations and demanded the remaining £1.5m be handed over "without further delay".

Mrs Kanabus said: "Jason admired the Prince of Wales for his support for farming and the Prince's Trust made sense for its support of young people. We can't believe they are not falling over backwards to make this work. The Prince's Trust is not the organisation we thought it was."